1. Field of the Invention
The present invention relates to an image processing apparatus and a control method therefor, and in particular to an image processing apparatus for reducing noise in image signals and a control method therefor.
2. Description of the Related Art
Image signals obtained with image sensors generally contain noise components. Among noise components, random noise generated temporally or spatially in random is generated mainly during high-sensitivity shooting. Especially, noise generated temporally in random is one of the factors that degrade the image quality of moving images.
As a random noise reduction technique for moving images, recursive noise reduction using correlation between moving image frames is known. For example, Japanese Patent Laid-open No. 2000-209507 discloses a technique of controlling a recursive coefficient in the recursive noise reduction based on a difference signal value between the image signal of the current frame and the image signal of the previous frame. A recursive coefficient is a coefficient indicating a proportion of components of a previous frame to be used in noise reduction on an image signal of a current frame. If the difference between the current and previous frames is large, afterimage is reduced by using a small recursive coefficient, and if the difference is small, noise is reduced by using a large recursive coefficient.
The method disclosed in Japanese Patent Laid-Open No. 2000-209507 controls the recursive coefficient based on a difference signal value, and so the computational complexity is small and the cost can be reduced. However, the accuracy of separation between random noise and moving components between frames is not enough, and therefore the image quality acquired after the noise reduction is insufficient.
Meanwhile, in Japanese Patent Laid-Open No. 2009-147822, temporal random noise is eliminated while afterimage of a moving object is suppressed by using a difference between the current frame that has been subjected to spatial noise reduction and one or more past frames (after being subjected to the spatial noise reduction). With the method disclosed in Japanese Patent Laid-Open No. 2009-147822, the accuracy of separation between random noise and moving components between frames is improved, and relatively high image quality can be obtained.
However, Japanese Patent Laid-Open No. 2009-147822 does not disclose noise reduction in which the image frame rate is considered. The temporal distance between frames (shooting time interval) varies depending on whether or not the image frame rate is high or low, and the degree of a correlation between consecutive frames is also affected. However, uniform noise reduction has been conventionally performed without considering frame rates. Accordingly, with the methods disclosed in Japanese Patent Laid-Open Nos. 2000-209507 and 2009-147822 sufficient effects cannot be achieved in the case where the image frame rate is variable.